Threads.synchronize() fails tensor copy to fail

[priyaradhakrishnan0]

I want to copy tensors in batches of 5. I created a thread each to copy the tensor. I call Threads.synchronize() to complete the end-callback (i.e copying) before further processing. But calling Threads.synchronize() always causes copy to fail. Is there any way to ensure end-callbacks is completed without calling synchronize()??

require("nn")
local threads = require 'threads'
local nthread = 4
local njob = 10
local batch_size = 5
local contexts_batch = torch.FloatTensor(batch_size,6):zero()

local pool = threads.Threads(
nthread,
function(threadid)
print('starting a new thread/state number ' .. threadid)
end
)
local c = 1
local jobdone = 0
for j=1,20 do
if c<= batch_size then
pool:addjob(
function()
local contexts = torch.FloatTensor(6)
contexts[1] = 2
local i = 0
while i < 5 do
contexts[i+2] = 1
i = i + 1
end
print (string.format('Contexts size %d, from task %d, thread ID is %x', contexts:size()[1], i,__threadid))
return contexts
end,

     function(contexts)
        print(string.format("Ending callback : task %d finished ", j ))
        contexts_batch[c]:copy(contexts)
        jobdone = jobdone + 1
     end
  )
  c = c+1

else
--pool:synchronize() <<<<This causes error
c = 1
end
end

pool:synchronize() <<<<This does not
print(string.format('%d jobs done', jobdone))
pool:terminate()

Error 👍
Ending callback : task 1 finished
Ending callback : task 2 finished
Ending callback : task 3 finished
Ending callback : task 4 finished
Ending callback : task 5 finished
/opt/torch/install/bin/luajit: /opt/torch/install/share/lua/5.1/threads/threads.lua:255:
[thread 1 endcallback] bad argument #2 to '?' (out of range at /opt/torch/pkg/torch/generic/Tensor.c:853)
[thread 2 endcallback] bad argument #2 to '?' (out of range at /opt/torch/pkg/torch/generic/Tensor.c:853)
[thread 1 endcallback] bad argument #2 to '?' (out of range at /opt/torch/pkg/torch/generic/Tensor.c:853)
[thread 2 endcallback] bad argument #2 to '?' (out of range at /opt/torch/pkg/torch/generic/Tensor.c:853)
[thread 1 endcallback] bad argument #2 to '?' (out of range at /opt/torch/pkg/torch/generic/Tensor.c:853)
stack traceback:
[C]: in function 'error'
/opt/torch/install/share/lua/5.1/threads/threads.lua:255: in function 'synchronize'